Jiuquan Satellite Launch Center is historically China's preeminent spaceport. It's located only about five degrees south of Baikonur Cosmodrome, whose location, I've been led to believe, was selected explicitly for the advantages in takeoff velocity that arise from launching nearer to the equator. NASA and the ESA have also constructed spaceports at the most tropical latitudes available to them (in the ESA's case, even going so far as to place their spaceport on another continent). So why, given that China has plenty of sparsely populated southern reaches, some of which currently host more recent spaceports, would they have located their primary launch facility so far north?
There are two embodied steps in the question a) "pre-eminent" which raises the question "for what purpose" and b) that "north" does not follow the pattern adopted by other participants.
Expanding a little:
a) That the crewed Shenzhou missions depart from Jiuquan might make it "pre-eminent" in some points of view. Please say if this isn't what you meant.
b) An "equatorial" launch site is preferable for the specific task of launching to geostationary transfer orbit (GTO). Cape canaveral and Vandenburg are, I believe historically sited because of existing missile ranges at the dawn of the space age. I believe Kourou was explicitly chosen for GTO access and the wide variety of launch azimuths that also permits polar launches.
All geostationary launches todate have been from Xichang (28 deg N) except a very recent demonstration mission on the CZ5 first launch. As the new Wenchang site is at 20 deg perhaps the future, with the newer launch vehicles, is that the GTO traffic will go from Wenchang.
Polar missions in general need a clear range North or South and there is a modest energy advantage in being away from the equator. I don't know the history of Jiquan (~40 N) and why it is that the manned missions have been launched from there though it is the oldest launch site and perhaps there are range and institutional reasons. I'm sure someone else on this forum will be able to expand on this.
Lastly, this is a helpful related Q&A.
EDIT - Background
This link mentions that Cape Canaveral was chosen over El Centro in California as a missile range because of a lack of international objection for the former for missile overflight.
This link suggests that Vandenburg was chosen in 1956 not only because it was remote but also for its access to the Pacific as a missile range and it also hints at some foresight for its polar orbit access.
This link describes the selection process that led to CSG at Kourou amongst 14 candidate sites globally. Its in French and is very interesting but this bit is probably easy enough to follow:
Critères de sélection :
Possibilité de lancements polaires et équatoriaux
Proximité de l'équateur
... and it goes on to list some further criteria and then to say that Kourou had positive characteristics in these areas.
- These two links NTI and Global Security whilst not explicitly giving the reason for launches being conducted from Jiquan do both describe the origins as a missile range. That leaves inductive reasoning in order to assume therefore that it was chosen as a missile range because of remoteness and that the launches began there simply because that's where the infrastructure already existed. It would be interesting to understand what launch azimuths have been attempted from this site given the number of near neighbouring countries.
An aside: - an interesting point about the advantage of being closer to the equator is that it is not just the rotation of the Earth that is of benefit but also in reducing the plane change for GEO bound satellites. For the GTO example (the vast majority of GEO bound satellites will start their missions at 0 deg inclination) the reduction in the cost of a plane change when one starts from 5 deg latitude (Kourou) instead of 28 deg (Cape Canaveral) is a better advantage than the assistance given by the tangential velocity at the Earth surface.
1$\begingroup$ Is the $\Delta v$ 'bonus' launching near the equator of benefit only for GTO, or would it be for any launch to low inclination near the limit of the launch vehicle, for example some missions to cis-lunar or deep space solar system? $\endgroup$– uhohApr 4, 2017 at 3:22
1$\begingroup$ You're correct that the exclusive use of Jiuquan for manned launches, along with total launches, total tenure, and highest payload launches were reasons for my conclusion for its "preeminence". $\endgroup$ Apr 4, 2017 at 6:35
2$\begingroup$ Also, this is a helpful response, which brings up a number of relevant issues, but does not directly answer the question: why was Jiuquan the selected location for establishing a spaceport? Unrelatedly, comments regarding Kuorou polar launches and Canaveral/Vandenberg missile ranges are intriguing, and I'd love to read more. Do you have citations? $\endgroup$ Apr 4, 2017 at 8:58
1$\begingroup$ @Mud Bungle Yes, a good point well made, I think I read the question too quickly and it doesn't at all get to the reasons for Jiuquan being where it is. For Kourou and the US launch sites I was basically going on what I've absorbed over the years. I've had a look around to see if I can find some references and added some example text to the answer. $\endgroup$– PuffinApr 4, 2017 at 20:57
2$\begingroup$ @uhoh I think your assertion makes sense - I've a GEO background so I am a bit out of safe territory with interplanetary missions though it sounds like a lot will come down to whether one has the opportunity to take advantage of the rotation given any other launch-window constraints. I've also added an aside to the answer: I think the plane change advantage that is specific to having a launch site with a latitude close to the target orbit inclination is worth rather more than the Earth's rotation speed bonus. $\endgroup$– PuffinApr 4, 2017 at 21:14